perf: Auto-detect libelf

Adds autodetection for libelf as well, and simplifies the
libbfd code. Furthermore, fail make with an error when libelf
is not found and warn about the lack of libbfd.

Also provide an option to build a 32bit version even though you
might be running a 64bit kernel.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>